The control of electrolytic cells in the production of aluminium is influenced by both short term and long term process parameter changes. In the short term, bath superheat, alumina concentration and anode to cathode distance (ACD) need constant monitoring, while longer term control is required for metal depth and the composition and volume of the electrolyte in the cell. Operating abnormalities also require attention, such as sludging, anode effects and their frequency, and the short circuiting of the current between the anodes and the metal pad.
The complexity of the interrelationships between the dependent and independent variables in the smelting process are illustrated in Chapter 9 of "Aluminium Smelter Technology"-Grjotheim and Welch-Aluminium-Verlag, 1988, and this chapter provides a useful summary of the currently utilised control strategies. This summary and the proliferation of literature on the subject further illustrate the complexity of the problem and the absence of a strategy that provides a satisfactory level of control resulting in constantly high efficiency levels.
Numerous examples of control strategy proposals are also to be found in the patent literature. Recent examples include U.S. Pat. No. 4,654,129 Leroy which describes a process involving periods of over supply and under supply to maintain the alumina concentration in the cell within a narrow range by monitoring the rate of change of the resistance of the cell. This process relies for its success on the use of point feeding of alumina to the cell, and it is not therefore useful for cells without point feeders. Also, since in this strategy it is critical to maintain the alumina concentration within a narrow range, the strategy suffers if the concentration moves outside that range and it is often difficult to restore the system to its optimum operating conditions.
A similar control strategy is described in International Patent Application PCT/NO86/00017 (W086/050008) Aalbu et al. In common with the above U.S. patent, the strategy relies heavily on the rate of change of the resistance of the cell to monitor alumina concentration and does not have regard to other important parameters to control the heat and mass balance of the cell. The disclosure similarly does not address the strategy to be adopted during process events, such as alumina feeding, anode movements, anode setting and tapping.
U.S. Pat. Nos. 4,008,142 and 4,024,034 Doring et al, uses the concept of constant anode-cathode distance to adjust cell resistance according to the known or assumed electrochemical voltage breakdown. Anode-cathode distance adjustment is made in cases where current efficiency (by metal production measurement) is less than expected theoretically. Automatic adjustment of voltage/cell resistance in response to noise on the signal is also indicated. However, no attempt is made to calculate the heat or alumina balances or to make furnace adjustments on this basis, with the exception of adjustment of cell resistance on the basis of long term running metal production figures. This does not constitute a calculation of the energy balance or process energy requirement.
In U.S. Pat. No. 4,766,552 Aalbu et al, the resistance/alumina concentration curve is used to control alumina concentration on point feed cells. A linear model of the cell resistance variation is set up using the resistance slope as a parameter. By fitting the model to continuous resistance measurements, the slope is estimated. However, this strategy does not ensure that the resulting slope is related only to alumina concentration, in fact it assumes this one to one relationship. Anode movement is included in the fitted algorithm and other disturbances are filtered by reducing the gain of the fitting functions when they occur. This procedure is very complex and could be prone to error. In addition, the strategy does not attempt to maintain heat balance within the cell.
In U.S. Pat. No. 4,333,803 Seger and Haupin, a heat flux sensor is used to measure sidewall heat flow. Cell resistance is adjusted to maintain this at a predetermined value. However, this strategy:
1. does not guarantee that heat losses from other portions of the cell are under control (top, bottom); PA1 2. does not react to changes inside the cell on a useful time scale (hours or within a day)--the cell can be significantly out of heat balance before an adjustment is made; and PA1 3. does not provide information about the events/operations occurring in the electrolyte. These events are needed to close the overall energy balance--including the continuous changing process requirements--and to sense the condition of the liquid electrolyte which is where electrolysis is taking place. Effective bath resistivity sensing in the strategy disclosed here allows much faster response to a heat imbalance in the electrolyte. PA1 (i) continuously monitoring cell voltage and current, PA1 (ii) calculating the resistance of the cell from the monitored cell voltage and current, PA1 (iii) calculating the rate of change of cell resistance (resistance slope) and providing a smoothed value of said resistance slope, PA1 (iv) utilizing the smoothed resistance slope values to maintain mass balance in the cell, PA1 (v) monitoring cell process operations including alumina additions, electrolyte bath additions, anode changes, tapping, beam raising and anode beam movement, PA1 (vi) delaying the calculation of resistance slope and smoothed resistance slope for a predetermined time when any one of said monitored cell process operations occurs, and PA1 (vii) recalculating said cell resistance slope and smoothed resistance slope after said predetermined time delay so that the smoothed slope is unaffected by process changes with the exception of alumina depletion. PA1 (a) monitoring the cell voltage and current and calculating the resistance of the cell from the monitored voltage and current, PA1 (b) monitoring alumina additions to the cell, monitoring other additions to the cell bath and monitoring operational changes including anode movements, tapping, anode setting and beam raising, PA1 (c) continuously calculating the energy absorbed by the process from thermodynamic energy requirements associated with the cell reaction and the events identified in item (b) above, PA1 (d) calculating the heat available for dissipation in the cell from the cell voltage and current and from the continuously calculated process energy requirement determined in item (c) above, PA1 (e) calculating from the calculated heat available for dissipation in (d) and from a selected target power dissipation, the integral of the difference between the heat available and the target power dissipation with respect to time to provide a running heat inventory or integral, PA1 (f) calculating from this heat deficit or surplus in the cell the change in power dissipation required in the cell over a predetermined period to restore heat balance (zero heat integral in item (e)), PA1 (g) establishing an initial target resistance for the cell and an allowable band for said target resistance, PA1 (h) Calculating the required change in target resistance from the required change in cell power dissipation (item (f)) divided by the square of a moving average of the monitored cell current, PA1 (i) altering the target resistance in accordance with the calculated heat inventory (item (e)) and checking that the new target resistance is within said allowable band, and PA1 (j) moving the anodes of the cell to achieve said new target resistance.
Other control strategies are described in U.S. Pat. Nos. 3,969,669 Brault and Lacroise, 3,829,365 Chandhuri et al, 4,431,491 Bonny et al, 4,654,129 Leroy, 4,654,130 Tabereaux et al, 3,622,475 Shiver, 3,878,070 Murphy, 3,573,179 Dirth et al, 4,035,251 Shiver and 4,488,117 Seo. This list is by no means intended to be exhaustive.
A primary factor in reduction cell efficiency is the thermal state of the materials in the cell cavity. A control strategy directed at optimizing efficiency should therefore aim to maintain a thermal steady state in the cell. That is, the rate of heat dissipation from the cell cavity should be kept constant. If this is achieved in concert with stable bath and metal inventories, operational stability can be enhanced. The bath superheat will be constant; hence bath volume, chemistry and temperature will be stable due to the absence of ledge freezing or melting. Improved operational stability may allow a cell to be operated with better alumina feed control, at a lower bath ratio, and at a lower time averaged rate of heat loss. This will improve the process productivity.
A major difficulty in maintaining thermal steady state in a reduction cell is the discontinuous nature of various operations. The energy requirements of alumina feeding and dissolution can vary from minute-to-minute, particularly on breaker-bar cells. This is further exacerbated by the deliberate changes in feed rate required by many feed control techniques. Anode setting in pre-baked cells also introduces a large cyclic energy requirement. Other processes, such as bath additions, anode effects and amperage fluctuations further alter the short-term thermal balance of a cell. Currently available control systems do not address these fluctuating thermal requirements in a comprehensive way. For example, target voltage control has allowed for alumina feeding in some systems. Similarly, anode effects have been used to control the power input. However, the complete range of variable energy requirements are not treated systematically or quantitatively to maintain a constant rate of heat supply available for dissipation through the cell.